Resonator device including electrode with buried temperature compensating layer

ABSTRACT

An acoustic resonator device includes a composite first electrode on a substrate, a piezoelectric layer on the composite electrode, and a second electrode on the piezoelectric layer. The first electrode includes a buried temperature compensating layer having a positive temperature coefficient. The piezoelectric layer has a negative temperature coefficient, and thus the positive temperature coefficient of the temperature compensating layer offsets at least a portion of the negative temperature coefficient of the piezoelectric layer.

BACKGROUND

Electrical resonators are widely incorporated in modern electronicdevices. For example, in wireless communications devices, radiofrequency (RF) and microwave frequency resonators are used as filters,such as ladder filters having electrically connected series and shuntresonators formed in a ladder structure. The filters may be included ina duplexer, for example, connected between a single antenna and areceiver and a transmitter for respectively filtering received andtransmitted signals.

Various types of filters use mechanical resonators, such as bulkacoustic wave (BAW), surface acoustic wave (SAW), and solidly mountedresonator (SMR)-BAW resonators. The resonators generally convertelectrical signals to mechanical signals or vibrations, and/ormechanical signals or vibrations to electrical signals. A BAW resonator,for example, is an acoustic stack that generally includes a layer ofpiezoelectric material between two electrodes. Acoustic waves achieveresonance across the acoustic stack, with the resonant frequency of thewaves being determined by the materials in the acoustic stack and thethickness of each layer (e.g., piezoelectric layer and electrodelayers). One type of BAW resonator includes a piezoelectric film as thepiezoelectric material, which may be referred to as a film bulk acousticresonator (FBAR). FBARs resonate at GHz frequencies, and are thusrelatively compact, having thicknesses on the order of microns andlength and width dimensions of hundreds of microns.

Resonators may be used as band-pass filters with associated passbandsproviding ranges of frequencies permitted to pass through the filters.The passbands of the resonator filters tend to shift in response toenvironmental and operational factors, such as changes in temperatureand/or incident power. For example, the passband of a resonator filtermoves lower in frequency in response to rising temperature and higherincident power.

Cellular phones, in particular, are negatively affected by shifts inpassband due to fluctuations in temperature and power. For example, acellular phone includes power amplifiers (PAs) that must be able to dealwith larger than expected insertion losses at the edges of the filter(duplexer). As the filter passband shifts down in frequency, e.g., dueto rising temperature, the point of maximum absorption of power in thefilter, which is designed to be above the passband, moves down into thefrequency range of the FCC or government designated passband. At thispoint, the filter begins to absorb more power from the PA and heats up,causing the temperature to increase further. Thus, the filter passbandshifts down in frequency more, bringing the maximum filter absorbingpoint even closer. This sets up a potential runaway situation, which isavoided only by the fact that the reflected power becomes large and thefilter eventually settles at some high temperature.

PAs are designed specifically to handle the worst case power handling ofthe filter at the corner of the pass band. Currents of a typical PA canrun from a few mA at the center of the filter passband to about 380mA-450 mA at the edges. This is a huge power draw on the PA, as well asthe battery that drives the cellular phone. This is one reason that acellular phone operating more in the transmit mode (i.e., talk time)than in the receive mode (i.e., listening time) drains battery powermore quickly.

In order to prevent or reduce rising temperatures, a conventional filtermay include a layer of oxide material within the piezoelectric layer ofthe acoustic stack. The oxide material has a positive temperaturecoefficient, which at least partially offsets the negative temperaturecoefficients of the metal electrodes and the piezoelectric material,respectively. For example, the oxide material may be placed in thecenter of the piezoelectric layer or at either end of the piezoelectriclayer between the electrodes. However, that the acoustic couplingcoefficient (kt²) of the resonator is greatly compromised by theaddition of oxide material to the piezoelectric layer. This is becausethe oxide material appears as a “dead” capacitor in series with theactive piezoelectric material dielectric. Further, the oxide materialmay contaminate the piezoelectric material. For example, when thepiezoelectric material is aluminum nitride (AlN), the oxide materialcauses the AlN to become a chemical compound that includes oxygen (e.g.,AlN_((x))O_((y))), which is a non-piezoelectric material, thus furtherdegrading the acoustic coupling coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion. Wherever applicable and practical, like referencenumerals refer to like elements.

FIG. 1 is a cross-sectional diagram illustrating an acoustic resonatordevice, including an electrode with a buried temperature compensatinglayer, according to a representative embodiment.

FIG. 2 is a cross-sectional diagram illustrating an acoustic resonatordevice, including an electrode with a buried temperature compensatinglayer, according to a representative embodiment.

FIG. 3 is a flow diagram illustrating a method of fabricating anacoustic resonator device, according to a representative embodiment.

FIG. 4 is a cross-sectional diagram illustrating an acoustic resonatordevice, including an electrode with a buried temperature compensatinglayer, according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, representative embodiments disclosing specific detailsare set forth in order to provide a thorough understanding of thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparatuses andmethods may be omitted so as to not obscure the description of therepresentative embodiments. Such methods and apparatuses are clearlywithin the scope of the present teachings.

Generally, it is understood that the drawings and the various elementsdepicted therein are not drawn to scale. Further, relative terms, suchas “above,” “below,” “top,” “bottom,” “upper,” “lower,” “left,” “right,”“vertical” and “horizontal,” are used to describe the various elements'relationships to one another, as illustrated in the accompanyingdrawings. It is understood that these relative terms are intended toencompass different orientations of the device and/or elements inaddition to the orientation depicted in the drawings. For example, ifthe device were inverted with respect to the view in the drawings, anelement described as “above” another element, for example, would now be“below” that element. Likewise, if the device were rotated 90 degreeswith respect to the view in the drawings, an element described as“vertical,” for example, would now be “horizontal.”

Aspects of the present teachings are relevant to components of BAW andFBAR devices and filters, their materials and their methods offabrication. Various details of such devices and corresponding methodsof fabrication may be found, for example, in one or more of thefollowing U.S. patent publications: U.S. Pat. No. 6,107,721, to Lakin;U.S. Pat. Nos. 5,587,620, 5,873,153, 6,507,983, 7,388,454 and 7,629,865to Ruby et al.; U.S. Pat. No. 7,280,007 to Hongjun Feng et al.; and U.S.Patent Application Pub. No. 2007-0205850 to Jamneala et al. Thedisclosures of these patents and published patent applications arehereby incorporated by reference. It is emphasized that the components,materials and method of fabrication described in these patents andpatent applications are representative and other methods of fabricationand materials within the purview of one of ordinary skill in the art arecontemplated.

According to various embodiments, a resonator device has an acousticstack with a piezoelectric layer between top and bottom electrodes, atleast one of which is a composite electrode having a temperaturecompensating layer deposited between an electrode layer and a conductiveinterposer layer. The temperature compensating layer may be formed of anoxide material, such as boron silicate glass (BSG), for example, havinga positive temperature coefficient which offsets at least a portion ofthe negative temperature coefficients of the piezoelectric layer and theconductive material in the top and bottom electrodes. The conductiveinterposer layer thus makes a DC electrical connection with theelectrode layer in the composite electrode, effectively shorting out acapacitive component of the temperature compensating layer andincreasing a coupling coefficient kt² of the resonator device. Also, theconductive interposer, which is positioned between the temperaturecompensating layer the piezoelectric layer, presents a barrierpreventing oxygen in the oxide layer from diffusing into thepiezoelectric material of the piezoelectric layer. In variousembodiments, the composite electrode may be the bottom electrode, thetop electrode, or both, in the acoustic stack.

FIG. 1 is a cross-sectional view of an acoustic resonator device, whichincludes an electrode having a buried temperature compensating layer,according to a representative embodiment.

Referring to FIG. 1, illustrative resonator device 100 includes acousticstack 105 formed on substrate 110. The substrate 110 may be formed ofvarious types of semiconductor materials compatible with semiconductorprocesses, such as silicon (Si), gallium arsenide (GaAs), indiumphosphide (InP), or the like, which is useful for integratingconnections and electronics, thus reducing size and cost. In thedepicted embodiment, the substrate 110 includes a cavity 115 formedbeneath the acoustic stack 105 to provide acoustic isolation, such thatthe acoustic stack 105 is suspended over an air space to enablemechanical movement. In alternative embodiments, the substrate 110 maybe formed with no cavity 115, for example, using SMR technology. Forexample, the acoustic stack 105 may be formed over an acoustic mirror ora Bragg Reflector (not shown), having alternating layers of high and lowacoustic impedance materials, formed in the substrate 110. An acousticreflector may be fabricated according to various techniques, an exampleof which is described in U.S. Pat. No. 7,358,831 to Larson, III, et al.,which is hereby incorporated by reference.

The acoustic stack 105 includes piezoelectric layer 130 formed betweenfirst electrode 120 and second electrode 140. The first electrode 120includes multiple layers, and is referred to herein as a compositeelectrode. In various embodiments, the composite first electrode 120includes a base electrode layer 122, a buried temperature compensatinglayer, e.g., buried oxide layer 124, and a conductive interposer layer126 stacked sequentially on the substrate 110. The base electrode layer122 and the conductive interposer layer 126 are formed of electricallyconductive materials, such as various metals compatible withsemiconductor processes, including tungsten (W), molybdenum (Mo),aluminum (Al), platinum (Pt), ruthenium (Ru), niobium (Nb), or hafnium(Hf), for example.

In various embodiments, the base electrode layer 122 and the conductiveinterposer layer 126 are formed of different conductive materials, wherethe base electrode layer 122 is formed of a material having relativelylower conductivity and relatively higher acoustic impedance, and theconductive interposer layer 126 is formed of a material havingrelatively higher conductivity and relatively lower acoustic impedance.For example, the base electrode layer 122 may be formed of W and theconductive interposer layer 126 may be formed of Mo, although othermaterials and/or combinations of materials may be used without departingfrom the scope of the present teachings. Further, in variousembodiments, the base electrode layer 122 and the conductive interposerlayer 126 may be formed of the same conductive material, withoutdeparting from the scope of the present teachings.

The buried oxide layer 124 is a temperature compensating layer, and isformed between the base electrode layer 122 and the conductiveinterposer layer 126. The buried oxide layer 124 is therefore separatedor isolated from the piezoelectric layer 130 by the conductiveinterposer layer 126, and is otherwise sealed in by the connectionbetween the conductive interposer layer 126 and the base electrode layer122. Accordingly, the buried oxide layer 124 is effectively buriedwithin the composite first electrode 120. The buried oxide layer 124 maybe formed of various materials compatible with semiconductor processes,including boron silicate glass (BSG), silicon dioxide (SiO₂), chromium(Cr) or tellurium oxide (TeO_((x))), for example, which have positivetemperature coefficients. The positive temperature coefficient of theburied oxide layer 124 offsets negative temperature coefficients ofother materials in the acoustic stack 105, including the piezoelectriclayer 130, the second base electrode 140, and the base electrode layer122 and the conductive interposer layer 126 of the composite firstelectrode 120.

As shown in the embodiment of FIG. 1, the buried oxide layer 124 doesnot extend the full width of the acoustic stack 105. Thus, theconductive interposer layer 126, which is formed on the top and sidesurfaces of the buried oxide layer 124, contacts the top surface of thebase electrode layer 122, as indicated for example by reference number129. Therefore, a DC electrical connection is formed between theconductive interposer layer 126 and the base electrode layer 122. By DCelectrically connecting with the base electrode layer 122, theconductive interposer layer 126 effectively “shorts” out a capacitivecomponent of the buried oxide layer 124, thus increasing a couplingcoefficient (kt²) of the resonator device 100. In addition, theconductive interposer layer 126 provides a barrier that prevents oxygenin the buried oxide layer 124 from diffusing into the piezoelectriclayer 130, preventing contamination of the piezoelectric layer 130.

Also, in the depicted embodiment, the buried oxide layer 124 has taperededges 124 a, which enhance the DC electrical connection between theconductive interposer layer 126 and the base electrode layer 122. Inaddition, the tapered edges 124 a enhance the mechanical connectionbetween the conductive interposer layer 126 and the base electrode layer122, which improves the sealing quality, e.g., for preventing oxygen inthe buried oxide layer 124 from diffusing into the piezoelectric layer130. In alternative embodiments, the edges of the buried oxide layer 124are not tapered, but may be substantially perpendicular to the top andbottom surfaces of the buried oxide layer 124, for example, withoutdeparting from the scope of the present teachings.

The piezoelectric layer 130 is formed on the top surface of theconductive interposer layer 126. The piezoelectric layer 130 may beformed of a thin film piezoelectric compatible with semiconductorprocesses, such as aluminum nitride (AlN), zinc oxide (ZnO), leadzirconium titanate (PZT), or the like. The thickness of thepiezoelectric layer 130 may range from about 1000 Å to about 100,000 Å,for example, although the thickness may vary to provide unique benefitsfor any particular situation or to meet application specific designrequirements of various implementations, as would be apparent to one ofordinary skill in the art. In an embodiment, the piezoelectric layer 130may be formed on a seed layer (not shown) disposed over an upper surfacethe composite first electrode 120. For example, the seed layer may beformed of Al to foster growth of an AlN piezoelectric layer 130. Theseed layer may have a thickness in the range of about 50 Å to about 5000Å, for example.

The second electrode 140 is formed on the top surface of thepiezoelectric layer 130. The second electrode 140 is formed of anelectrically conductive material compatible with semiconductorprocesses, such as Mo, W, Al, Pt, Ru, Nb, Hf, or the like. In anembodiment, the second electrode 140 is formed of the same material asthe base electrode layer 122 of the composite first electrode 120.However, in various embodiments, the second electrode 140 may be formedof the same material as only the conductive interposer layer 126; thesecond electrode 140, the conductive interposer layer 126 and the baseelectrode layer 122 may all be formed of the same material; or thesecond electrode 140 may be formed of a different material than both theconductive interposer layer 126 and the base electrode layer 122,without departing from the scope of the present teachings.

The second electrode 140 may further include a passivation layer (notshown), which may be formed of various types of materials, includingAlN, silicon carbide (SiC), BSG, SiO₂, SiN, polysilicon, and the like.The thickness of the passivation layer must be sufficient to insulateall layers of the acoustic stack 105 from the environment, includingprotection from moisture, corrosives, contaminants, debris and the like.The first and second electrodes 120 and 140 are electrically connectedto external circuitry via contact pads (not shown), which may be formedof a conductive material, such as gold, gold-tin alloy or the like.

In an embodiment, an overall first thickness T₁₂₀ of the first electrode120 is substantially the same as an overall second thickness T₁₄₀ of thesecond electrode 140, as shown in FIG. 1. For example, the thickness ofeach of the first and second electrodes 120 and 140 may range from about600 Å to about 30000 Å, although the thicknesses may vary to provideunique benefits for any particular situation or to meet applicationspecific design requirements of various implementations, as would beapparent to one of ordinary skill in the art.

The multiple layers of the composite first electrode 120 havecorresponding thicknesses. For example, the thickness of base electrodelayer 122 may range from about 400 Å to about 29,900 Å, the thickness ofburied oxide layer 124 may range from about 100 Å to about 5000 Å, andthe thickness of conductive interposer layer 126 may range from about100 Å to about 10000 Å. Each of the layers of the composite firstelectrode 120 may be varied to produce different characteristics withrespect to temperature coefficients and coupling coefficients, while theoverall first thickness T₁₂₀ of the composite first electrode 120remains substantially the same as the overall second thickness T₁₄₀ ofthe second electrode 140. For example, the thickness of the buried oxidelayer 124 may be varied to affect the overall temperature coefficient ofthe acoustic stack 105, and the relative thicknesses of the baseelectrode layer 122 and the conductive interposer layer 126 may bevaried to affect the overall coupling coefficient of the resonatordevice 100.

For example, FIG. 2 depicts a cross-sectional view of an acousticresonator device, according to another representative embodiment, inwhich thicknesses of electrode and conductive interposer layers arevaried, thus “sinking” a buried temperature compensating layer deeperinto a composite first electrode (and further away from the activepiezoelectric layer 130).

More particularly, illustrative resonator device 200 of FIG. 2 includesacoustic stack 205 formed on substrate 110. The acoustic stack 205includes piezoelectric layer 130 formed between a composite firstelectrode 220 and a second electrode 140. Like reference numerals inFIGS. 1 and 2 refer to like elements, and therefore correspondingdescriptions of like elements will not be repeated.

The composite first electrode 220 includes electrode layer 222, buriedoxide layer 224 and conductive interposer layer 226 stacked sequentiallyon the substrate 110, e.g., over the cavity 115. As discussed above withrespect to the base electrode layer 122 and the conductive interposerlayer 126, the electrode layer 222 and the conductive interposer layer226 are formed of the same or different electrically conductivematerials, such as Mo, W, Al, Pt, Ru, Nb or Hf, for example.

The buried oxide layer 224 is a temperature compensating layer formedbetween the electrode layer 222 and the conductive interposer layer 226,such that the buried oxide layer 224 is separated from the piezoelectriclayer 130 by the conductive interposer layer 226. Accordingly, theburied oxide layer 224 is effectively buried within the composite firstelectrode 220. As discussed above with respect to the buried oxide layer124, the buried oxide layer 224 may be formed of various materialscompatible with semiconductor processes, including BSG, SiO₂, Cr orTeO_((x)), for example, which have positive temperature coefficients,for offsetting negative temperature coefficients of other materials inthe acoustic stack 205.

As discussed above with reference to FIG. 1, the overall thickness ofthe composite first electrode 220 is substantially the same as theoverall thickness of the second electrode 140, as indicated by firstthickness T₂₂₀ and second thickness T₁₄₀. However, the thickness T₂₂₆ ofthe conductive interposer layer 226 in FIG. 2 is greater than thethickness T₁₂₆ of the conductive interposer layer 126 in FIG. 1, suchthat the buried oxide layer 224 has been buried more deeply, i.e.,further “sinking,” within the composite first electrode 220 than withinthe composite first electrode 120. To compensate for the greaterthickness T₂₂₆ of the conductive interposer layer 226, the thickness ofthe electrode layer 222 in FIG. 2 is less than the thickness T₁₂₂ of thebase electrode layer 122 in FIG. 1, so that the overall first thicknessT₂₂₀ of the composite first electrode 220 remains the same as theoverall second thickness T₁₄₀ of the second electrode 140.

The thickness of the oxide layer can also be targeted to be thicker (asit is more deeply buried) to help maintain, or minimize, the lineartemperature coefficient. In the depicted example, the thickness of theburied oxide layer 224 is the same as the thickness of the buried oxidelayer 124. However, due to the deeper position within the compositefirst electrode 220, the buried oxide layer 224 causes the couplingcoefficient of the resonator device 200 to be relatively greater thanthe coupling coefficient of the resonator device 100 (at the expense ofworsening temperature coefficient). In other words, by adjusting thedepth of the buried oxide layer 124, 224, the coupling coefficient ofthe resonator device 100, 200 may be optimized. Some of the degradationof the temperature coefficient can be “won back” by thickening theburied oxide layer 124, 224. Typically, there is an optimum betweenfinal temperature coefficient and coupling coefficient (kt²), dependingon application.

Generally, the thickness and the location of the buried oxide layer 124,224 inside the composite first electrode 120, 220 should be optimized,in order to maximize the coupling coefficient for an allowable lineartemperature coefficient. This optimization may be accomplished, forexample, by modeling an equivalent circuit of the acoustic stack 105,205 using a Mason model and adjusting the buried oxide layer 124, 224 byadding more material to the conductive interposer layer 126 and removingmaterial from the base electrode layer 122, so the thickness of thecomposite first electrode 120, 220 remains constant, as would beapparent to one of ordinary skill in the art. Although there is somedegradation in the offsetting effects of the temperature coefficient bysinking the buried oxide layer 124, 224, the coupling coefficient of theresonator device 100, 200 may be improved. An algorithm may be developedto optimize the depth of the buried oxide layer 124, 224 in thecomposite first electrode 120, 220 in light of the trade-off between thetemperature coefficient and the coupling coefficient, for example, usinga multivariate optimization technique, such as a Simplex method, aswould be apparent to one of ordinary skill in the art. In addition, thedepth of the buried oxide layer 124, 224, may be limited by variousconstraints, such as minimum necessary coupling coefficient and maximumallowable temperature coefficient. Likewise, the thickness of the buriedoxide layer 124, 224 may adjusted to provide the optimal couplingcoefficient and a minimum overall temperature coefficient of theresonator device 100, 200.

Referring again to FIG. 1, in an illustrative configuration of theacoustic stack 105, the buried oxide layer 124 is formed at a thicknessof about 1000 Å using a thin film of BSG (e.g., about two percent byweight boron), which provides a large positive temperature coefficient(e.g., up to about 350 ppm per deg C). Each of the first thickness T₁₂₀of the first electrode 120 and the second thickness T₁₄₀ of the secondelectrode 140 (including a passivation layer) is about 3000 Å. Also, thebase electrode layer 122 of the first electrode 120 and the secondelectrode 140 are each formed of Mo. The conductive interposer layer 126is also made of molybdenum, and in this example would be between about300 Å and about 600 Å. The piezoelectric layer 130 is formed at athickness of about 11,000 Å using a thin film of AlN. The acoustic stack105 with this illustrative configuration has a zero linear temperaturecoefficient value. Only a residual quadratic term is left (where beta isabout −22 ppB per degree C²). However, the maximum coupling coefficientfor the resonator device 100 of this configuration is only about fourpercent. In comparison, when the BSG layer is formed outside of theacoustic stack 105 (as in some conventional resonators), the temperaturecoefficient has a linear term on the order of about −24 ppm per degreeC. and a coupling coefficient that should be on the order of 6.5percent.

According to various embodiments, the resonator device may be fabricatedusing various techniques compatible with semiconductor processes. Anon-limiting example of a fabrication process directed to representativeresonator device 100 is discussed below with reference to FIG. 3.

FIG. 3 is a flow diagram illustrating a method of fabricating aresonator device, according to a representative embodiment.

Referring to FIGS. 1 and 3, substrate 110 is provided in block 5311 andthe base electrode layer 122 is applied to a top surface of thesubstrate 110 in block 5312. In an embodiment, the substrate 110 isformed of Si and the base electrode layer 122 is formed of W, forexample, although different materials may be used, as discussed above,without departing from the scope of the present teachings. The baseelectrode layer 122 may be applied using spin-on, sputtering,evaporation or chemical vapor disposition (CVD) techniques, for example,although other application methods may be incorporated.

Notably, formation of the cavity 115 in the substrate 110 may be carriedout before fabrication of the acoustic stack 105, wherein the cavity 115is initially filled with a sacrificial material (not shown), such asphosphosilicate glass (PSG) or other release processes, such aspolysilicon and xenon difluoride etchant, as would be apparent to one ofordinary skill in the art, during fabrication of layers of the acousticstack 105. The release of the sacrificial material to form the cavity115 is carried out using a suitable etchant, such as HF, afterfabrication of the layers of the acoustic stack 105 (e.g., afterformation of the second electrode 140). In alternative configurations,the cavity 115 may pass through the substrate 110 to form a backsideopening, which may be formed by back side etching a bottom surface ofthe substrate 110. The back side etching may include a dry etch process,such as a Bosch process, for example, although various alternativetechniques may be incorporated.

Alternatively, the substrate 110 may include an acoustic isolator, suchas an acoustic mirror or Bragg Reflectors, rather than the cavity 115.Such acoustic isolator may be formed in the substrate 110 using anytechnique compatible with semiconductor processes before forming theacoustic stack 105, as would be apparent to one of ordinary skill in theart.

In block S313, buried oxide layer 124 is formed on a top surface of thebase electrode layer 122, to form a temperature compensating layer. Inan embodiment, the buried oxide layer 124 is formed of BSG, for example,although different materials may be used, as discussed above, withoutdeparting from the scope of the present teachings. The buried oxidelayer 124 may be applied using spin-on, sputtering, evaporation or CVDtechniques, for example, although other application methods may beincorporated. Various illustrative techniques for forming temperaturecompensating layers are described, for example, in U.S. Pat. No.7,561,009 to Larson, III, et al., which is hereby incorporated byreference.

In block S314, the buried oxide layer 124 is etched to a desired sizeand the tapered edges 124 a are tapered. For example, a photoresistlayer (not shown) may be applied to the top surface of the buried oxidelayer 124 and patterned to form a mask or photoresist pattern, using anyphostoresist patterning technique compatible with semiconductorprocesses, as would be apparent to one of ordinary skill in the art. Thephotoresist pattern may be formed by machining or by chemically etchingthe photoresist layer using photolithography, although variousalternative techniques may be incorporated. Following etching of theburied oxide layer 124, the photoresist pattern is removed, for example,by chemically releasing or etching using a wet etch process including HFetch solution, although the photoresist pattern may be removed byvarious other techniques, without departing from the scope of thepresent teachings.

In various embodiments, to obtain the tapered edges 124 a, oxygen isleaked into the etcher used to etch the buried oxide layer 124. Theoxide (and/or temperature chuck) causes the photoresist to erode morequickly at the edges of the patterned photo resist and to pull backslightly. This “thinning” of the resist forms a wedge shape profile thatis then imprinted into the oxide underneath as the photoresist goesaway. Generally, the wedge is created by adjusting the etch rate ofresist relative to the etched material, as would be apparent to one ofordinary skill in the art. Meanwhile, further from the edges of theburied oxide layer 124, there is sufficient photoresist coveragethroughout the etch that the underlying oxide material is not touched.Of course, other methods of obtaining tapered edges may be incorporatedwithout departing form the scope of the present teachings.

The conductive interposer layer 126 is applied to a top surface of theburied oxide layer 124 in block S315. The conductive interposer layer126 is formed of Mo, for example, although different materials may beused, as discussed above, without departing from the scope of thepresent teachings. The conductive interposer layer 126 may be appliedusing spin-on, sputtering, evaporation or CVD techniques, for example,although other application methods may be incorporated.

In an alternative embodiment, an interim seed layer (not shown) isformed on the top surface of the buried oxide layer 124 before theburied oxide layer 124 is etched. The interim seed layer may be formedof the same piezoelectric material as the piezoelectric layer 130, suchas MN, for example. The interim seed layer may be formed to a thicknessof about 300 Å, for example, and reduces or minimizes oxide diffusionfrom the oxide layer 124 into the piezoelectric layer 130. Outerportions of the interim seed layer are removed by etching, along withthe etched portions of the buried oxide layer 124, to expose portions ofthe top surface of the base electrode layer 122, so that the baseelectrode layer 122 is able to make an electrical connection betweenwith the conductive interposer layer 126. In other words, after etching,the interim seed layer covers only the top surface of the buried oxidelayer 124, so that it is positioned between the buried oxide layer 124and the conductive interposer layer 126.

In block S316, the piezoelectric layer 130 is applied to a top surfaceof the conductive interposer layer 126, which is also the top surface ofthe first electrode 120. The piezoelectric layer 130 is formed of AlN,for example, although different materials may be used, as discussedabove, without departing from the scope of the present teachings. Thepiezoelectric layer 130 may be applied using a sputtering technique, forexample, although other application methods may be incorporated. Forexample, the piezoelectric layer 130 may be grown from a seed layer, asdiscussed above, according to various techniques compatible withsemiconductor processes.

The second electrode 140 is applied to a top surface of thepiezoelectric layer 130 in block S317. The is second electrode 140formed of W, for example, although different materials may be used, asdiscussed above, without departing from the scope of the presentteachings. The second electrode 140 may be applied using spin-on,sputtering, evaporation or CVD techniques, for example, although otherapplication methods may be incorporated. In various embodiments, thesecond electrode 140 includes a passivation layer formed of BSG, SiO₂,SiN, polysilicon, or the like.

The resonator device 100 may then be cut or separated from a wafer, tothe extent necessary, in order to form a singulated die. The resonatordevice 100 may be separated using various techniques compatible withsemiconductor fabrication processes, such as scribe and break, forexample.

FIG. 4 is a cross-sectional view of an acoustic resonator device, whichincludes an electrode having a buried temperature compensating layer,according to a representative embodiment, in which the acoustic stack ofthe resonator device is reversed, such that the second (top) electrodeis a composite electrode, as opposed to the first (bottom) electrode.

Referring to FIG. 4, illustrative resonator device 400 includes acousticstack 405 formed on substrate 410. The substrate 410 may be formed ofvarious types of semiconductor materials, Si, GaAs, InP, or the like,and may include a cavity 415 or an acoustic isolator, as discussed abovewith respect to substrate 110 in FIG. 1. The acoustic stack 405 includespiezoelectric layer 430 formed between a first electrode 420 and acomposite second electrode 440.

More particularly, the first electrode 420 is formed of an electricallyconductive material, such as W, Mo, Al, Pt, Ru, Nb or Hf, for example,on the substrate 410. The piezoelectric layer 430 is formed of apiezoelectric material, such as AlN, ZnO or PZT, for example, on thefirst electrode 420. The composite second electrode 440 is formed on thepiezoelectric layer 430, such that a buried oxide layer 444 is separatedfrom the piezoelectric layer 430 by a conductive interposer layer 446.

For example, in an embodiment, the conductive interposer layer 446 isapplied to the top, substantially planar surface of the piezoelectriclayer 430 at a desired thickness T₄₄₆. As discussed above, the thickerthe conductive interposer layer 446, the more buried the buried oxidelayer 444 is within the composite second electrode 440 (i.e., furtherremoved from the piezoelectric layer 430). The buried oxide layer 444 isthen applied to the top surface of the conductive interposer layer 446to form a temperature compensating layer. The buried oxide layer 444 maybe applied using spin-on, sputtering, evaporation or CVD techniques, forexample, although other application methods may be incorporated. Also,the buried oxide layer 444 is etched to a desired size and the edges 444a may be tapered, as discussed above with respect to the buried oxidelayer 124.

The electrode layer 442 is formed over the buried oxide layer 444 andthe conductive interposer layer 446. The electrode layer 442 has athickness T₄₄₂ (at outer portions, not over the buried oxide layer 444),which may include a passivation layer (not shown), as discussed above.The thickness T₄₄₂ may vary such that an overall second thickness T₄₄₀of the composite second electrode 440 is substantially the same as anoverall first thickness T₄₂₀ of the first electrode 420.

As discussed above, the conductive interposer layer 446 and theelectrode layer 442 are formed of electrically conductive materials,such W, Mo, Al, Pt, Ru, Nb or Hf, for example. Also, the conductiveinterposer layer 446 and the electrode layer 422 may be formed of thesame or different materials, to provide various benefits or to meetapplication specific design requirements of various implementations, aswould be apparent to one of ordinary skill in the art. In an embodiment,the electrode layer 442 is formed of the same material as the firstelectrode 420, although the electrode layer 442 and the first electrode420 may be formed of different materials from one another in alternativeembodiments.

The buried oxide layer 444 is a temperature compensating layer, formedbetween the conductive interposer layer 446 and the electrode layer 442,in substantially the same manner discussed above with respect to buriedoxide layer 124. The buried oxide layer 444 may be formed of variousmaterials compatible with semiconductor processes, including BSG, SiO₂,SiN, or polysilicon, for example, which have positive temperaturecoefficients. Because the buried oxide layer 444 does not extend thefull width of the acoustic stack 405, the conductive interposer layer446 forms a DC electrical connection with the electrode layer 442, whicheffectively “shorts” out a capacitive component of the buried oxidelayer 444 and increases a coupling coefficient (kt²) of the resonatorstack 400, as discussed above. In addition, the conductive interposerlayer 446 provides a barrier that prevents oxygen in the buried oxidelayer 444 from diffusing into the piezoelectric layer 430.

In various additional embodiments, the acoustic stack of the resonatordevice may include composite electrodes formed on both the top andbottom surfaces of the piezoelectric layer, without departing from thescope of the present teachings.

According to various embodiments, an acoustic stack of a resonatordevice has at least one composite electrode that includes a buriedtemperature compensating layer separated from a piezoelectric layer by aconductive interposer layer. The temperature compensating layer has atemperature coefficient that has an opposite sign from a temperaturecoefficient of at least one other element in the acoustic stack, thusoffsetting the effects of that temperature coefficient. Further, theconductive interposer layer effectively shorts out a capacitivecomponent of the temperature compensating layer, which effectivelyincreases a coupling coefficient of the resonator device. Accordingly,this enables more stable operation of the resonator, for example, bypreventing shifts in passband due to increases in temperature, whilepreventing contamination of the piezoelectric material by the materialin the temperature compensating layer.

The various components, materials, structures and parameters areincluded by way of illustration and example only and not in any limitingsense. In view of this disclosure, those skilled in the art canimplement the present teachings in determining their own applicationsand needed components, materials, structures and equipment to implementthese applications, while remaining within the scope of the appendedclaims.

The invention claimed is:
 1. An acoustic resonator device comprising: acomposite first electrode over a substrate, the first electrodecomprising first and second electrically conductive layers and a buriedtemperature compensating layer between the first and second electricallyconductive layers, the temperature compensating layer having a positivetemperature coefficient; a piezoelectric layer on the first electrode,the piezoelectric layer having a negative temperature coefficient; and asecond electrode on the piezoelectric layer, wherein the positivetemperature coefficient of the temperature compensating layer offsets atleast a portion of the negative temperature coefficient of thepiezoelectric layer, wherein the temperature compensating layercomprises tapered edges.
 2. The device of claim 1, wherein the firstelectrically conductive layer comprises a first metal layer formed onthe substrate; and the second electrically conductive layer comprises asecond metal layer formed on the temperature compensating layer.
 3. Thedevice of claim 2, wherein the temperature compensating layer comprisesan oxide material.
 4. The device of claim 2, wherein a depth of thetemperature compensating layer within the composite first electrode isdetermined by a thickness of the second metal layer relative to athickness of the first metal layer.
 5. The device of claim 4, whereinthe depth of the temperature compensating layer within the compositefirst electrode affects a coupling coefficient of the acoustic resonatordevice.
 6. The device of claim 3, wherein the oxide material comprisesboron silicate glass (BSG).
 7. The device of claim 3 wherein a thicknessof the temperature compensating layer is adjusted for an optimalcoupling coefficient and a minimum temperature coefficient of theresonator device.
 8. The device of claim 6, wherein the first metallayer comprises tungsten and the second metal layer comprisesmolybdenum, and wherein the second electrode comprises tungsten.
 9. Thedevice of claim 1, wherein the second electrode has a negativetemperature coefficient, the positive temperature coefficient of thetemperature compensating layer further offsetting at least a portion ofthe negative temperature coefficient of the second electrode in additionto offsetting at least the portion of the negative temperaturecoefficient of the piezoelectric layer.
 10. The device of claim 1,wherein the substrate defines a cavity over which the first electrode ispositioned.
 11. The device of claim 1, wherein the substrate includes anacoustic mirror over which the first electrode is positioned.
 12. Afilter having a passband for filtering at least one of transmitted orreceived radio frequency (RF) signals, the filter comprising: theacoustic resonator device of claim 1, wherein the buried temperaturecompensating layer comprises a buried boron silicate glass (BSG) layer,and the first electrically conductive layer connects with the secondelectrically conductive layer making a DC electrical contact andisolating the BSG layer from the piezoelectric layer.
 13. The filter ofclaim 12, wherein the positive temperature coefficient of the BSG layeroffsets the at least a portion of the negative temperature coefficientof the piezoelectric layer, reducing shifts in the passband due toincreases in operational temperature of the filter.
 14. An acousticresonator device comprising: a composite first electrode on a substrate,the first electrode comprising first and second electrically conductivelayers and a buried temperature compensating layer between the first andsecond electrically conductive layers, the temperature compensatinglayer having a positive temperature coefficient; a piezoelectric layeron the first electrode, the piezoelectric layer having a negativetemperature coefficient; and a second electrode on the piezoelectriclayer, wherein the positive temperature coefficient of the temperaturecompensating layer offsets at least a portion of the negativetemperature coefficient of the piezoelectric layer, wherein thetemperature compensating layer comprises an oxide material, and whereinthe first electrode further comprises an interim seed layer formedbetween the temperature compensating layer and the second metal layer,the interim seed layer reducing oxide diffusion from the temperaturecompensating layer into the piezoelectric layer.
 15. An acousticresonator device comprising: a composite first electrode over asubstrate, the first electrode comprising first and second electricallyconductive layers and a buried temperature compensating layer betweenthe first and second electrically conductive layers, the temperaturecompensating layer having a positive temperature coefficient; apiezoelectric layer on the first electrode, the piezoelectric layerhaving a negative temperature coefficient; and a second electrode on thepiezoelectric layer, wherein the positive temperature coefficient of thetemperature compensating layer offsets at least a portion of thenegative temperature coefficient of the piezoelectric layer, wherein thesecond electrically conductive layer forms an electrical contact withthe first electrically conductive layer on at least one side of thetemperature compensating layer, the electrical contact electricallyshorting a capacitive component of the temperature compensating layer.16. The device of claim 15, wherein the temperature compensating layerincludes tapered edges.
 17. An acoustic resonator device comprising: asubstrate; a first electrode formed on the substrate; a piezoelectriclayer formed on the first electrode, the piezoelectric layer having anegative temperature coefficient; and a second electrode formed on thepiezoelectric layer, wherein at least one of the first and secondelectrodes is a composite electrode comprising: a buried oxide layerhaving a positive temperature coefficient offsetting at least a portionof the negative temperature coefficient of the piezoelectric layer; aconductive interposer layer formed between the oxide layer and thepiezoelectric layer; and a base electrode layer formed on an oppositeside of the oxide layer from the conductive interposer layer, the baseelectrode layer contacting the conductive interposer layer to form aseal around the oxide layer, isolating the oxide layer from thepiezoelectric layer.
 18. The device of claim 17, wherein the baseelectrode layer contacting the conductive interposer layer furtherelectrically connects the base electrode layer and the conductiveinterposer layer, effectively shorting out a capacitive component of theoxide layer and increasing a coupling coefficient of the resonatordevice.
 19. The device of claim 17, wherein only one of the first andsecond electrodes is the composite electrode.
 20. The device of claim17, wherein both the first and second electrodes are compositeelectrodes.
 21. An acoustic resonator device, comprising: a substrate; acomposite electrode disposed over the substrate, the composite electrodecomprising first and second electrically conductive layers and atemperature compensating layer disposed between the first and secondelectrically conductive layers, wherein the second electricallyconductive layer forms an electrical contact with the first electricallyconductive layer on at least one side of the temperature compensatinglayer, the electrical contact electrically shorting a capacitivecomponent of the temperature compensating layer; a piezoelectric layeradjacent to the composite electrode; an acoustic reflector disposedbeneath the composite electrode; and an interim seed layer formedbetween the temperature compensating layer and the second metal layer,the interim seed layer reducing oxide diffusion from the temperaturecompensating layer into the piezoelectric layer.
 22. The acousticresonator device of claim 21, wherein the first electrically conductivelayer comprises a first metal layer; and the second electricallyconductive layer comprises a second metal layer disposed on thetemperature compensating layer.
 23. The acoustic resonator device ofclaim 22, wherein the temperature compensating layer comprises an oxidematerial.
 24. The acoustic resonator device of claim 23, wherein theoxide material comprises boron silicate glass (BSG).
 25. The acousticresonator device of claim 23, wherein a thickness of the temperaturecompensating layer is adjusted for an optimal coupling coefficient and aminimum temperature coefficient of the resonator device.
 26. The deviceof claim 24, wherein the first metal layer comprises tungsten and thesecond metal layer comprises molybdenum, and wherein the secondelectrode comprises tungsten.
 27. The acoustic resonator device of claim22, wherein a depth of the temperature compensating layer within thecomposite first electrode is determined by a thickness of the secondmetal layer relative to a thickness of the first metal layer.
 28. Theacoustic resonator device of claim 27, wherein the depth of thetemperature compensating layer within the composite first electrodeaffects a coupling coefficient of the acoustic resonator device.